The present invention relates to methods for removing sludge from bodies of water, e.g., wastewater lagoons.
Wastewater from chemical processes, e.g., paper manufacture, has long been treated by directing the wastewater through a hold/release basin or "lagoon" in which solids suspended in the wastewater settle, forming a sludge on the bottom of the lagoon, and discharging the water into a river or ocean. Wastewater typically flows into the lagoon continuously, or periodically on a continuous basis, and is discharged continuously or periodically, e.g., based on the tides if the lagoon is located in a coastal region.
Both the time which the wastewater spends in the lagoon prior to discharge (retention time) and the velocity of the wastewater as it passes through the lagoon affect the amount of solids which are able to settle, and thus the amount of solids which are discharged with the water. As sludge builds up on the bottom of the lagoon, the retention time of the lagoon is decreased and the velocity of wastewater through the lagoon increased, causing less of the suspended solids to settle prior to discharge. The buildup of sludge will thus eventually cause the suspended solids in the discharged wastewater to exceed acceptable levels. This causes a particular problem in applications where the suspended solids are hazardous to the environment, e.g., dioxin-containing solids generated in paper processing.
It is thus necessary to periodically remove sludge from the bottom of the lagoon, if the lagoon is to remain in use indefinitely. Removal has been attempted in the past by simple dredging, i.e., by pumping sludge up from the bottom of the lagoon. Dredging has proved unsatisfactory for use in many wastewater processing lagoons, however, as large quantities of sludge are stirred up from the bottom of the lagoon and resuspended in the water, contaminating the entire lagoon and resulting in unacceptable levels of suspended solids in the discharged water.